Superconducting MRI Magnet Research Articles

Mechanical Stress Analysis of Zero-Boil-Off Cryostat for a 1.5 T MRI Magnet

Abstract A whole-body 1.5 T superconducting MRI magnet is operated in 4.2 K inside a zero-boil-off helium cryogenic vessel, i.e., cryostat. The cryostat has horizontal bore of 850 mm used as the patient bore. The cryostat houses the 1.5 T superconducting magnet in a coaxial position maintaining a high degree of alignment along with the gradient coil, the birdcage RF coil necessary for any MRI scanner. A set of support link having low thermal conductivity and higher mechanical strength would provide structural support to the cold mass at 4.2 K, which includes the superconducting magnet, helium vessel. A two stage 4.2 K G-M cryocooler is used to recondense helium to achieve a “zero boil-off” condition. The helium vessel and the vacuum vessel of the cryostat experience stresses during the normal operation, transportation etc. The ASME B&PV Code and the Finite Element Analysis has been used for the engineering design and stress analysis of cryostat. The modal analysis of the cryostat has been done during the transportation of the cryostat. The seismic behavior of the cryostat has also been analyzed extensively for a 1.5 T MRI cryostat.

Quench Induced Eddy Current and Mechanical Stresses in the Bobbin and Thermal Shield of the 1.5 T MRI Magnet System

An actively shielded 1.5 T superconducting MRI magnet is designed for a whole-body clinical scanner. During a quench, its fast-decaying field induces a strong eddy current in all the conductive parts of the cryostat. The eddy current together with the decaying magnetic field generates Lorentz force, which generates stress and deformation in the bobbin structure, thermal shield, and other conductive parts of the cryostat. In this article, we report the analysis of the eddy current distribution induced in the bobbin structure and thermal shield made of various alloys of the MRI magnet system using a finite element analysis program. The net forces on each part of the bobbin structure and the thermal shield and the associated stress distribution, the deformation are analyzed in detail using OPERA-3D software. The eddy-current-induced effects on a few metallic alloys are compared for generating design inputs for the bobbin structure and the thermal shield of the MRI magnet system. We also report the transient behavior of the eddy current in correlation with the decaying current during a quench.

Quench Protection System of a Whole-Body Non-impregnated 1.5 T Superconducting MRI Magnet

A quench protection system for an actively-shielded whole-body 1.5 T MRI magnet made with wire-in-channel NbTi conductor is presented with two types of coil sub-divisions. The MRI magnet must operate for many years in the hospital, and thereby, it must be completely self-protecting by means of self-activating quench propagation heaters to spread the normal zone in a larger volume of the magnet during a quench i.e. the passive protection system. The total stored energy of the magnet is about 3.7 MJ. The voltage and the temperature of the coils have been simulated with two types of passive quench protection systems using OPERA-Quench program. The maximum inter-layer quench voltage generated in the asymmetric sub-division is 70 V/layer with a peak voltage of 2.6 kV and a hot spot temperature of 248 K. The maximum inter-layer quench voltage generated in the symmetric sub-division is 45 V/layer with a peak voltage of 1.7 kV and a hot spot temperature of 185 K. During a quench, an average driving voltage of 1.3 kV and the associated current of 1.75 A is generated in both quench propagation circuits of the magnet with the symmetric sub-division. The 5 G safety line explodes beyond the permissible limits during a quench of the magnet with the asymmetric sub-division, whereas the 5 G safety region is retained within the permissible limit in case of quench of the magnet with symmetric sub-division.

Thermal and Electrical Behaviour of the Persistent Current Switch for a Whole-Body Superconducting MRI Magnet

A prototype persistent current switch is developed for an actively shielded whole-body 1.5 T MRI magnet having an operating current of ∼500 A. The switch is developed using a six-strand CuNi-NbTi conductor. The total length of the conductor used in the PCS is ∼40 m using a bifilar winding technique having a room temperature resistance of 15 Ω and an inductance of 6.6 μH. Two numbers of thermo-foil heaters having a resistance of 90 Ω each are placed between the layers of the winding pack of the switch. The wet-winding technique is followed for the switch using a cryogenic grade epoxy. The characteristics of the switch are performed using a 4 K test rig for its applicability in the 1.5 T MRI magnet. The normal resistance of the switch is measured to be 12.5 Ω at 15 K which is 15% less than the estimated value. The total energy loss onto the switch is estimated to be 0.24% of the ramping energy of the magnet at 6 V charging voltage. The thermal switching profile of the switch is studied and correlated with the total energy loss.

Magnetic modeling of actively shielded rotating MRI magnets in the presence of environmental steel

Rotating MRI systems could enable novel integrated medical devices such as MRI-Linacs, MRI-xray-angiography systems, and MRI-proton therapy systems. This work aimed to investigate the feasibility of rotating actively shielded superconducting MRI magnets in the presence of environmental steel—in particular, construction steel in the floor of the installation site. Two magnets were investigated: a 1.0 T split bore magnet, and a 1.5 T closed bore magnet. Each magnet was scaled to emulate field strengths of 0.5, 1.0, and 1.5 T. Finite Element Modeling was used to simulate these magnets in the presence of a 3 × 4 m steel plate located 1250 mm or 1400 mm below the isocenter. There are two possible rotation directions: around the longitudinal (z) axis or around the transverse (x) axis. Each model was solved for rotation angles between 0 and 360° in 30° intervals around each of these axes. For each simulation, a 300 mm DSV was extracted and decomposed into spherical harmonics. For the closed-bore magnet, total induced perturbation for the zero degree rotation angle was 223, 432, and 562 μT peak-to-peak (pk–pk) for the 0.5, 1.0, and 1.5 T models respectively (steel at 1250 mm). For the split-bore magnet, the same numbers were 1477, 16747, and 1766 μT. The substantially higher perturbation for the split-bore magnet can be traced to its larger fringe field. For rotation around the z-axis, total perturbation does not change as a function of angle but is exchanged between different harmonics. For rotation around the x-axis, total perturbation is different at each rotation angle. For the closed bore magnet, maximum perturbations occurred for a 90° rotation around the transverse axis. For the split-bore magnet, the opposite was observed, with the same 90° rotation yielding total perturbation lower than the conventional position. In all cases, at least 95% of the total perturbation was composed of 1st and 2nd order harmonics. The presence of environmental steel poses a major challenge to the realization of an actively shielded rotating superconducting MRI system, requiring some novel form of shimming. Possible shimming strategies are discussed at length.

Technology challenges of 14 T whole-body superconducting MRI magnets —A target of high-field superconducting magnet technology for large scale applications in next decade

This paper presents a brief review of the development trend of superconducting magnets in large scale applications towards high magnetic fields, depending on and pushing the Nb<sub>3</sub>Sn wire technics' continuous improvement. The focus is on analysis of the technology challenges of 14 T whole-body superconducting magnets. Using the Bonze Nb<sub>3</sub>Sn wires and on the base of a combination design of Nb<sub>3</sub>Sn and NbTi coils, an electromagnetic conception design of a 14 T whole-body MRI magnet is presented, and the thermal stability and quench protection are analyzed by simulations. The critical issues on stress, joints as well as shimming of 14 T whole-body superconducting magnets are also discussed. According to the results, this paper believes: 1) Nb<sub>3</sub>Sn wires are of the first important issue for 14 T whole-body superconducting magnets—the Bonze Nb<sub>3</sub>Sn wire is of the best choice but the performance specifications of the current products need to be improved further to match the requirements; 2) quench protection of 14 T whole-body superconducting magnets is one of the most complicated technics that covers design of the copper to superconductor (Cu/SC) ratio, coordination of the operating current and coil inductances, subdivisions of passive protection circuits and quench triggering control of active protection, as well as the stray field limitation during the transient process.

Helium Re-condenser for 1.5T Zero-Boil-Off (ZBO) MRI cryostat

Superconducting MRI magnet is the most exhaustive commercial application of superconductivity and helium cryogenics. Cryocooler based Zero-Boil-Off (ZBO) technique is inevitable for the present-day scanner. An MRI cryostat for a 1.5T MRI magnet has been designed with the ZBO technique using a fin-based helium recondenser. A cryocooler based test rig has been developed to characterize the performance of the helium recondenser made of OFHC copper. A two-stage GM cryocooler is used to cool the thermal shield and the helium recondenser. Four liters of liquid helium has been produced using the same heat exchanger. The recondensation test of the heat exchanger is performed up to 6 psi of bath pressure. At 1psi, the recondensation capacity is found to be ~1.5W. This paper briefly discusses the experimental details of the recondensation test along with an analytical calculation.

Stress-Induced Magnetic Field Inhomogeneity in a 1.5 T Superconducting MRI Magnet

The heart of the present day MRI scanners is a superconducting magnet producing the required central field. A 1.5 T multicoil magnet with ±5.5 ppm field homogeneity in 45 cm field of view (FOV) is designed. These coils will operate at an average current density of 130 A/mm 2 . The magnet bobbin will be made of 5083 aluminium alloy, while the winding pack of the magnet will be a composite structure which will consist of Nb-Ti, copper, epoxy and insulations. The winding pretension, thermal strains, due to cooling, and Lorentz forces contribute to the overall stress, which during operation can cause deformation of the magnet. To design a structurally sound and stable magnet system, it is critical to evaluate the stresses that are generated in the magnet during winding, cool down and operation. This paper discusses the stresses generated on the magnet and deformations produced due to the forces acting on the coils. The effect of deformation on the field homogeneity and the influence of winding tension on the overall operational stress are also studied. The von-Mises stress during the operation is found to be 40 MPa along the median line. The deformations of the magnet due to operational stresses increased the field homogeneity to ±85 ppm in the FOV.

Analysis of mechanical stress due to winding pretension in a 1.5 T superconducting MRI magnet

The paper presents the analysis of stresses due to winding tension in a 1.5 T MRI magnet with a clear bore of ∼1 m and an outer diameter of ∼1.8 m. The magnet has four pairs of symmetric coils arranged over a length of ∼1.5 m. The stringent requirement of field homogeneity makes it critical to design a structurally stable magnet system. It is essential to evaluate the 3-dimensional stresses that would be generated in the magnet during winding, cool down and operation. The bobbin structure also has to be designed so as to reduce the relative movements of theeight superconducting coils, which otherwise can adversely affect the field homogeneity at the Field of View (FOV). The bobbin will be made of aluminium 5083 alloy, while the magnet winding will be a composite structure which consists of NbTi, copper and polyester insulation. The paper discusses the stress distribution over the aluminium bobbin and the coil caused due to winding tension applied during magnet winding. The article also discusses the element death and birth method in FEA to accurately simulate the winding process.

Stress analysis of cryogenic suspension system of superconducting MRI magnet cryostat

Stress analysis of cryogenic suspension system of superconducting MRI magnet cryostat

Magnetic Field Stability Control of HTS-MRI Magnet by Use of Highly Stabilized Power Supply

Low-temperature superconducting MRI magnets are usually cooled by a large amount of liquid helium (LHe); however, LHe is getting scarcer and its price has been rising. Therefore, the development of helium free high-temperature superconducting (HTS)-MRI magnet is expected. HTS-MRI magnet fabricated by REBCO tape superconductors has mainly two problems for producing a highly stabilized magnetic field for MRI imaging. One problem is current fluctuation of a power supply, which is necessary for excitation because the permanent current operation of HTS-MRI magnet is difficult. Another is a screening current in REBCO tapes, which affect the magnetic field stability. The MRI magnetic field feedback control test was carried out, that is, the overshooting and the current feedback control, using 32 H HTS-MRI test magnet and the highly stabilized power supply. The magnetic field was evaluated under the current feedback control at 1.5 T (66 A). By the overshooting and the current feedback control, the magnetic field stability was improved to 0.7 ppm/ h which is enough for MRI imaging.

WE-G-17A-08: Electron Gun Operation for in Line MRI-Linac Configurations: An Assessment of Beam Fidelity and Recovery Techniques for Different SIDs and Magnetic Field Strengths

Purpose: To test the functionality of medical electron guns within the fringe field of a purpose built superconducting MRI magnet, and to test different recovery techniques for a variety of imaging field strengths and SIDs. Methods: Three different electron guns were simulated using Finite Element Modelling; a standard diode gun, a standard triode gun, and a novel diode gun designed to operate within parallel magnetic fields. The approximate working regime of each gun was established by assessing exit current in constant magnetic fields of varying strength and defining ‘working’ as less than 10% change in injection current. Next, the 1.0T MRI magnet was simulated within Comsol Multiphysics. The coil currents in this model were also scaled to produce field strengths of .5, 1, 1.5 and 3T. Various magnetic shield configurations were simulated, varying the SID from 800 to 1300mm. The average magnetic field within the gun region was assessed together with the distortion in the imaging volume - greater than 150uT distortion was considered unacceptable. Results: The conventional guns functioned in fields of less than 7.5mT. Conversely, the redesigned diode required fields greater than .1T to function correctly. Magnetic shielding was feasible for SIDS of greater than 1000mm for fieldmore » strengths of .5T and 1T, and 1100mm for 1.5 and 3.0T. Beyond these limits shielding resulted in unacceptable MRI distortion. In contrast, the redesigned diode could perform acceptably for SIDs of less than 812, 896, 931, and 974mm for imaging strengths of 0.5, 1.0, 1.5, 3.0T. Conclusions: For in-line MRIlinac configurations where the electron gun is operating in low field regions, shielding is a straight forward option. However, as magnetic field strength increases and the SID is reduced, shielding results in too great a distortion in the MRI and redesigning the electron optics is the preferable solution. The authors would like to acknowledge funding from the National Health and Research Council (AUS), National Institute of Health (NIH), and Cancer Institute NSW.« less

Optimization in design of actively shielded whole-body open high-field superconducting MRI magnet

We propose an optimization design method for actively shielded whole-body open high-field superconducting MRI magnet which accordingly has been simplified to contain only four pairs of superconducting coils. The new design method of open structure superconducting MRI magnet integrates the linear programming algorithm and the genetic algorithm optimization. Through several times of linear programming, and by taking into account the magnetic field, inhomogeneity in DSV, the scope of 5-Gauss fringe field, maximum hoop stress and maximum magnetic field, we can, with the least consumption of lines, get the coils’ initial position and shape, the number of layers of each coil and the number of turns of each layer. And the genetic algorithm was then employed to improve the magnetic field inhomogeneity in DSV to meet the requirements of high-quality imaging. This integrated optimization design method is flexible and effective for designing both open MRI magnet and traditional cylindrical MRI magnet. This paper also illustrates the method for a 1.2 T open MRI magnet optimization design.

Quench Protection Design of a 1.5 T Superconducting MRI Magnet

A 1.5 T superconducting MRI magnet has been developed in our laboratory. A passive quench protection system is employed to avoid the damage through the quench event. The coils are subdivided into several groups and a heater network is implemented accordingly. With the control volume method, the numerical model of the quench time is introduced. Different design schemes of the heater strip are compared. The simulation results of currents and voltages are illustrated and the temperature rise of the coils and the heaters are discussed.

Development of a Novel Hybrid Protection System for Superconducting MRI Magnets

In order to reliably protect the superconducting MRI magnets as soon as possible, a hybrid protection system is developed. The protection system, which combines the both merits of active and passive protection schemes, includes a quench detection controller, a protection superconducting switch, a capacitor bank, a strip heaters network and a protection circuitry with back to back diode sets and shunting resistance coils. The quench detection controller is employed to detect the start of a quench and to subsequently switch on the capacitor bank to quench the protection superconducting switch. The shunting current in the coil subdivision flows through the shunting resistance coil also to quench the protection superconducting switch. The voltage across the protection superconducting switch energizes the strip heaters. Once a quench occurs in a magnet, the protection system immediately initiates all superconducting coils to quench. The hybrid protection system is successfully applied to a 1.5 T whole-body MRI magnet. This paper describes the details of the hybrid protection system and presents the quench simulation results.

Frictional Behavior of Material Couples in Superconducting MRI Magnet Systems at 4.2 K

Windings in superconducting magnets operating at 4.2 K are highly susceptible to quenching caused by small frictional heat inputs. Small movements in the magnet system are inevitable during ramping due to the increasing electromagnetic forces. Friction pairs of polymer based materials have been investigated at 4.2 K to gain an understanding of their sliding behavior in conditions representative of superconducting MRI systems. The results indicate that polymer-polymer pairs experience unstable sliding behavior with repeated stick-slip whereas polymer-aluminum couples have stable sliding behavior up to high contact pressures of 20 MPa.

An optimizing design method for a compact iron shielded superconducting magnet for usein MRI**This work is supported partly by the Natural Science Foundation of China (no. 60871001).

A method is developed for designing a special iron shielded superconducting magnet forMRI in this paper. The shield is designed as an integral part of the cryostat and highpermeability and high saturated magnetization iron material is adopted. This scheme willresult in a compact iron shielded magnet. In the presented design, the finite element (FE)method is adopted to calculate the magnetic field produced by superconducting coils andnonlinear iron material. The FE method is incorporated into the simulated annealingmethod which is employed for corresponding optimization. Therefore, geometricalconfigurations of both coils and iron shield can be optimized together. This method candeal with discrete design variables which are defined to describe the cable arrangements ofcoil cross sections. A detailed algorithm of the present design is described and anexample for designing a 1.5 T clinical iron shielded magnet for MRI is shown.

An Active Quench Protection System for MRI Magnets

An active quench protection system is developed to safely protect a 1.5 T superconducting MRI magnet. The protection system includes a digital quench detection controller, a set of quench heaters and voltage taps, a battery backed DC power supply and solid-state switches. The quench controller takes the voltage signals from the voltage taps over superconducting coils and monitor a quench onset. To distinguish a quench signal from operational signals, the quench controller uses a pre-determined moving average voltage over 50 ms as the threshold signal for quench detection. The threshold signal is more than 10 times larger than any of the normal operation signals to avoid false quenching. Once detected moving average voltage exceeds the threshold, the quench controller will close solid-state relays and power quench heaters to accelerate the quench propagation over the entire magnet. The system has been tested in a MRI magnet test module.

Multiple Layer Superconducting Magnet Design for Magnetic Resonance Imaging

A design method of multiple layers superconducting magnet for MRI is introduced. Considering the actual coils layout, the candidate domain is divided into several layers. The current density curves of each layer are solved by Quadratic Programming optimization with Minimum Stored Energy. The seed coils are arranged at the peak positions of current curves. Based on the regularization solution, the current of the seed coils can be solved. According to the coil's current, selecting the current density of the superconducting wires, the original sections for the coils can be obtained. Then a further optimization about the homogeneity based on constrained nonlinear multivariable optimization method is employed to determine the final coils' geometries. The method is especially suitable for short superconducting MRI magnet design.

${\rm MgB}_{2}$ Coils for MRI Applications

In this paper, we report our progress in the development of a low cost, liquid-helium-free MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconducting MRI magnet. Several technical issues related to the construction of such a magnet to operate in persistent current mode are discussed, namely, the performance consistency of long length MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> conductors, the fabrication of superconducting joints and persistent current switches. Quench detection and protection of such magnets operated in solid nitrogen environment are also discussed.